PHOTOSYNTHETIC MACROMOLECULES OF CHLOROBIUM THIOSUI.FATOPHILUM 309 



of 2"o in a medium buffered at pH 7-4 and corresponding to the culture 

 medium except for the omission of reducing substances. The specimens 

 are dehydrated in a graded series of alcohols with the temperature pro- 

 gressively reduced to — 50 \ then the specimens are infiltrated with butyl 

 methacrylate monomer and polymerized by gamma radiation from a 

 cobalt-60 source at -50. In sections through an axial plane (Fig. 2) the 

 peripheral envelope, or cell wall, is distinct from the cytoplasmic mem- 

 brane. The "nucleus" is represented by the axial region of low electron 

 densitv which contains ramifying spiral filaments. Typically, there are one 

 or more relatively large circular areas of high electron density which 



300 



400 



500 



600 



700 



800 



900 



Fig. I. Illustration of the range of variation in content of accessory pigments 

 in cultures of Chlorohiiim thiosulfatophilum. The absorption spectra were measured 

 (Gary Model 14) through opal glass to reduce the effect of light scattering. 



represent sections through inclusions rich in polyphosphates. The cyto- 

 plasmic region has a stippled appearance owing to the presence of large 

 numbers of small particles. The lack of contrast between the particles and 

 the background prevents accurate determinations of size and form but it 

 is clear that the images are circular rather than elongated and have a 

 maximum extension of about 150 A. There is no indication ot the vescular 

 chromatophores (Fig. 3) which characterize Chromatiiim [6, 7, 39]), 

 RliodospiriUiim rubrum [22, 25, 39], Rhodopseiidomonas sphevoides [39], and 

 Chlorohiiim Umicola [39] of the peripheral lamellae which have been or, 

 described in Rliodomicrohium vaniiielii [40], or of the lamellated inclusions 

 of Rhodospiyillum moUsdiianiim [16]. In fact, the above description, 



